This invention relates to a process that facilitates the generation of syngas and/or hydrogen in a chemical reactor that includes an oxygen transport membrane. More particularly, the structural designs of the membrane and of a reactor housing that membrane are simplified by purging the anode side of the oxygen transport membrane with steam thereby reducing the pressure differential from the cathode side to the anode side of the oxygen transport membrane. Further benefits of the process are achieved by conducting a methane steam reforming reaction and a direct partial oxidation of methane reaction in a single reaction vessel that is separated from the oxygen transport membrane.
Oil and petrochemical companies have discovered vast quantities of natural gas in remote locations such as in polar regions and under seas. While the natural gas provides an inexpensive source of energy such as for the generation of steam, transport of the natural gas, which consists mostly of methane, is difficult. Typically, the methane is converted to synthesis gas (syngas), an intermediate in the conversion of natural gas to liquid fuels that are much more readily transported. Syngas is a mixture of hydrogen and carbon monoxide with an H2/CO molar ratio of from about 0.6 to about 6.
Reactions useful for the conversion of methane to syngas include a methane steam reforming process and a direct partial oxidation of methane. The methane steam reforming reaction:
CH4+H2Oxe2x86x92CO+3H2
Is an endothermic reaction having a relatively high yield of hydrogen gas (H2), producing three moles of hydrogen gas for each mole of carbon monoxide produced. The reaction kinetics require the addition of significant amounts of heat.
The direct partial oxidation of methane:
CH4+2O2xe2x86x92CO+2H2
is exothermic and generates two moles of hydrogen for each mole of carbon monoxide produced. The direct partial oxidation reaction further requires a source of oxygen. Air is generally an inefficient source of oxygen for the direct partial oxidation reaction because the high percentage of nitrogen contained within air acts as a diluent, significantly reducing the efficiency of the reaction and requiring subsequent separation from the synthesis gas product.
One source of oxygen for the direct partial oxidation reaction is through the use of an ionic or mixed conducting membrane reactor. A solid electrolyte membrane that has oxygen selectivity is disposed between an oxygen containing feed stream and an oxygen consuming, typically methane-containing product stream. xe2x80x9cOxygen selectivityxe2x80x9d means that oxygen ions are transported across the membrane while other elements, and ions thereof, are not. The solid electrolyte membrane is typically made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconia and analogous oxides, often having a fluorite or a perovskite structure.
At elevated temperatures, typically in excess of 500xc2x0 C., and preferably in the range of 700xc2x0 C.-1200xc2x0 C., the solid electrolyte membranes contain mobile oxygen ion vacancies that provide conduction sites for the selective transport of oxygen ions through the material. Because the membranes allow only oxygen transport, they function as a membrane with infinite selectivity for oxygen and are therefore very attractive for use in air separation processes.
Recognizing that the methane steam reforming reaction is endothermic and that the direct partial oxidation of methane reaction is exothermic, it would be attractive to combine the two reactions in a single reaction vessel. Combining the reactions adjacent to the oxygen transport membrane leads to a number of problems. The ceramic membranes are subject to steam corrosion and tend to lose oxygen from their lattice structure at the low partial oxygen pressures at the anode. High levels of stress are generated by differential compositional expansion in the membrane at the steep oxygen gradients across the membrane. The membrane can also become coated with coke and carbon reducing the effective transport of oxygen. It is also necessary to manage the heats of reaction to avoid hot spots along the membrane surface and to seal the membrane elements against high pressure differentials.
An alternate approach is to separate the oxygen separation membrane from the syngas generating reactions and to transport separated oxygen from the anode side of the oxygen transport membrane to a separate reactor for transacting the syngas generating reactions. A dual reactor approach avoids many of the problems mentioned above, however, a process that is less capital intensive is required.
U.S. Pat. No. 5,035,726 discloses the separation of argon from air utilizing an oxygen transport membrane to remove oxygen from the air. A subsequent distillation removes the nitrogen. To decrease the oxygen partial pressure on the anode side of the oxygen transport membrane, a sweep gas is employed. Waste nitrogen from distillation may be heated and utilized as the sweep gas.
U.S. Pat. No. 5,562,754 discloses an oxygen transport membrane employing a sweep gas on the anode side. The sweep gas may be steam at a temperature of between 800xc2x0 F. and 2000xc2x0 F. and at a pressure of between 2 psia and 300 psia. The steam is formed by heating boiler feed water in a combustor.
U.S. Pat. No. 5,964,922 discloses the use of steam as a purge gas on the anode side of an oxygen transport membrane. A mixture of steam and oxygen is injected into a reaction vessel remote from the oxygen transport membrane and utilized for coal gasification.
U.S. Pat. Nos. 5,035,726, 5,562,754 and 5,964,922 are each incorporated by reference in their entireties herein.
There remains, however, a need for a process to combine oxygen separation by a high temperature membrane with a syngas and/or hydrogen generating reactor that utilizes both an endothermic steam reforming reaction and an exothermic partial oxidation reaction in which the combination of reactions does not detrimentally affect the structural integrity of the oxygen transport membrane. There is further a need for a reactor to support the combination of reactions that is not capital intensive.
In accordance with a first preferred embodiment of the invention, a process to separate oxygen from an oxygen containing feed gas includes two in-line combustors.
In one aspect of the process, the process includes the following steps: (1) providing a first reactor housing an oxygen transport membrane that has a cathode side and an opposing anode side. This oxygen transport membrane is at a temperature that is effective to enable the transport of oxygen ions from the cathode side to the anode side. (2) Heating the oxygen containing feed gas in a first combustor and by recuperative heat exchange and then contacting the cathode side of the oxygen transport membrane with the heated oxygen containing feed gas. The oxygen containing feed gas having been compressed to a pressure of from 20 psia to 100 psia. (3) Heating a sweep gas that is, volumetrically, or on a molar basis, predominantly steam and has a steam pressure of from 10 psia to 30 psia and then contacting the anode side of the oxygen transport membrane with the heated sweep gas. Preferentially the sweep steam is generated by waste heat from the reaction process. (4) Recovering as anode effluent a mixture of transported oxygen and steam from the first reactor wherein the heated sweep gas volume is regulated to maintain an anode effluent with a steam to oxygen molar ratio that is in excess of 1:1. (5) Condensing and separating out steam contained in the anode effluent and compressing the resultant oxygen stream to partial oxidation or autothermal reactor pressure. (6) Heating the compressed oxygen gas and feeding it to the partial oxidation reactor or autothermal reformer.
In a first preferred aspect of this first embodiment, the process includes pre-heating the oxygen containing feed gas in a recuperative heat exchanger to a temperature that is within 100xc2x0 C. to 200xc2x0 C. of an intended temperature of the oxygen containing feed gas when exiting the first combustor. A typical intended temperature is between 800xc2x0 C. and 1000xc2x0 C. This first combustor may be fired with a mixture of a fuel and a minor portion of oxygen from the oxygen containing feed gas. In similar fashion, a portion of oxygen from the anode effluent may be mixed with a fuel and combusted in the second combustor to heat the sweep gas.
In a second preferred aspect of this first embodiment, both the first combustor and the second combustor are housed within the first reactor.
In a third preferred aspect of this first embodiment, the sweep steam is generated by using the latent heat of condensation of the steam contained in the reactor output stream and the available sensible heat in the resulting steam condensate.
In a fourth preferred aspect of this first embodiment, the reacting step in the second reactor includes both partial oxidation and steam reforming. One suitable second reactor is an auto-thermal reformer.
In accordance with a second embodiment of the invention the compressed air stream and the sweep steam are indirectly heated to membrane reactor operating temperature in a fired heater where the oxygen for the combustion is taken from the residual heat contained in the retentate stream effluent from the cathode side of the membrane reactor.
In accordance with a third embodiment of the invention, the sweep steam is taken from the exhaust of a steam turbine, the preferred inlet conditions for which are selected to enable the steam turbine to at least drive one of the air compressor or the oxygen compressor.
In a first preferred aspect of this third embodiment, the oxygen containing feed gas is heated in a first combustor and the sweep gas in a second combustor before contacting the oxygen containing feed gas and the sweep gas with the oxygen transport membrane. The first combustor may be fired with a mixture of a fuel and oxygen from the oxygen containing feed gas and the second combustor with a mixture of a minor portion of the oxygen from the anode effluent and a fuel.
In a second preferred aspect of this third embodiment, both the first combustor and the second combustor are housed within the first reactor.
In a third preferred aspect of this third embodiment, the steam for driving the steam turbine is generated by at least a portion of the sensible heat contained in the second reactor effluent.